In recent years diesel engines have been progressively replacing steam turbines in marine vessels, mainly as a result of the improved economics of the marine diesel. Marine diesel engines may generally be classified as slow speed, medium speed or high speed engines with the slow speed variety being used for major, deep draft vessels. Slow speed diesel engines are typically direct coupled, direct reversing, engines operating in the range of 90 to 250 rpm which are usually run on residual fuels. These engines are cross-head construction with a diaphragm and stuffing boxes separating the power cylinders from the crank case to prevent combustion products from entering the crankcase and mixing with the crankcase oil. Medium speed engines typically operate in the range of 250 to 1100 rpm and may operate on the four stroke or two stroke cycle. These engines operate well on intermediate fuel oil (180 cSt at 50.degree. C.) as compared to the heavier marine fuel oil (Bunker C) which is satisfactory for slow speed engines. High speed diesel engines comparable to automotive diesel engines are normally employed in deep draft vessels only for special, ancillary applications and these engines require high quality distillate fuel oil for satisfactory operation although recent developments in engine design have indicated that efficient operation on heavy fuels in feasible.
With low speed marine engines, the cylinders and crank case are lubricated separately, with cylinder lubrication being provided on a once-through basis by means of injection devices which apply cylinder oil to lubricators positioned around the cylinder liner. The crankcase oil provides lubrication for the bearings, gearing, valve gear and other ancillary engine components and is typically an additive type oil selected for good oxidation and thermal stability, water demulsability, corrosion protection and good antifoam performance. Alkaline additives may also be present to neutralize any strong acids entering the crankcase through piston rod glands and detergency and extreme pressure (EP) performance may also be provided by the use of suitable additives. Similar performance characteristics are appropriate for the crankcase oils in residual burning, medium speed engines in which the crankcase oil may also be used, in certain types, for splash cylinder lubrication. Other types of medium speed engines may have separate formed feed cylinder lubrication.
Intermingling of the crankcase oil and cylinder oil is generally unavoidable, particularly in view of the relatively large amounts of crankcase oil which may pass through the piston rod packing gland in large engines. The commingled oil is generally returned to the crankcase even though it may be comtaminated with cylinder oil drainings and blow-by gases. The potential for contamination makes it necessary, however, to monitor the condition of the crankcase oil at frequent intervals to ensure that satisfactory lubricating qualities are maintained.
Two specific problems frequently arising in marine diesel engines are blow-by of combustion products and raw fuel dilution of the lubricating oil. To a certain extent, blow-by is inevitable since some leakage around the piston rings takes place, especially in the larger engine sizes. Fuel dilution may occur when seals in engine ancillaries fail to perform adequately, with the result that the chemical composition of the lubricating oil may be altered and sludge formation accelerated with possible fouling of the engine and failure of major components. Normally, a certain degree of fuel dilution is considered acceptable and in most cases up to about 5% dilution can be tolerated. Frequent monitoring of the engine oil is, however, required to maintain fuel dilution within acceptable levels.
Another problem arising in the lubrication of marine diesel engines is sludge formation. The formation of sludge is obviously undesirable and it is very important to determine the cause of the sludge formation in order to prevent damage to the engine. This is particularly so with expensive marine engines. Sludge may be formed by fuel dilution of the lubricating oil because the lubricating oil is highly paraffinic whereas the fuel oils are rather aromatic materials which may be relatively immiscible with the lubricating oil. Sludges resulting from raw fuel dilution are probably the polar, highly aromatic fraction of the fuel oil called asphaltenes. This fraction should be only partly soluble in the lubricating oil. Finding the cause of the sludge formation is, however, difficult since the current practice is limited to elemental analysis and this is incapable of determining whether the cause of the sludge formation is related to fuel dilution.
So far, no conventional method for detecting fuel dilution of lubricating oils has been considered satisfactory. Viscosity measurements are unreliable. Chemical analysis of the vanadium content of the oil may also be misleading because it is incapable of distinguishing between vanadium derived from fuel dilution and vanadium which may have entered the lubricating oil as a result of blow-by. Although methods designed to separate fuel vanadium from combusted vanadium by extraction have been considered, they are time consuming and yield uncertain results. A need for a rapid analytical method for reliably detecting raw fuel dilution in marine lubricating oil therefore persists.